Many oil fields are comprised of reservoirs wherein the injection wells involving with a waterflood operation have or develop direct or indirect pressure communication with offset production wells. When this pressure communication develops, the producer well exhibits a breakthrough of injected water and thus an increase in the water-to-oil ratio and a decrease in oil production. The waterflood process may become effectively broken as a result. This breakthrough of injection water is a serious disadvantage which typically becomes more prevalent as an oil field matures.
Though there may be communication, the injector well may still hold injection pressure suggesting that the flooding operation of the reservoir has not completely failed. The pressure difference between the bottom hole injection well pressure and the bottom hole production well pressure can range up to thousands of pounds per square inch (psi). When communication occurs, this pressure differential will decrease. In severe instances, such as a matrix bypass event or “MBE” functional waterflood can completely fail which is signified by the bottom hole injection pressure of the injector becoming nearly equal to the bottom hole producing pressure of the producer.
Matrix bypass events (MBE's) are a particular problem in the waterflooding of many heavy/viscous oil reservoirs and are believed to develop gradually due to heavy oil production with sand from unconsolidated formations containing heavy oil. In some areas of the world, MBE's may be referred to as ‘worm holes’. The water injector well develops non-matrix communication with the oil production well. “Non-matrix” communication refers to the normal matrix pressure transient communication, which occurs in a functional waterflood between injector and producer via interconnected formation “matrix” pore spaces. When the matrix pore spaces between sand grains suddenly become replaced by a new form of pressure transient communication via a large open channel we call an MBE which has infinite ‘non-matrix’ conductivity of pressure acting between the wellbores. Thus, matrix sweep of oil ceases. The presence of an MBE can be inferred in most cases where the difference in pressure between the bottom hole injection well pressure and the bottom hole production well pressure (ΔPbh) shows a significant decrease over a relatively short period of time, such as a change in ΔPbh on the order of at least 100 psi over a 12 hour time period. However, it is not uncommon to see at least a several hundred psi (ΔPbh) swing in less than an hour on MBE's. In the most severe MBE cases, the final pressure differential (ΔPbh) between injector and producer may become less than 200 psi and even less than 100 psi, i.e., the bottom hole injection pressure of the water injector approaches and nearly equals the bottom hole production pressure of the producer. This short circuit of the functional waterflood can render continued injection ineffective and halts oil recovery, since the injector and producer pair will now only cycle water through the infinitely conductive MBE pathway.
Several methods exist for sealing MBE's. One such method involves the placement of solid granular materials (i.e., sand, silts, calcium carbonate, etc.) pumped as slurry into the MBE in an attempt to fill the MBE with new matrix material to seal the MBE. These materials are pumped either “full bore” directly down the tubing typically in a gelled carrying fluid as a slurry, or may be pumped down coil tubing to the MBE as a slurry. The disadvantage of this method is that there is no matrix cementation material to hold the solid particles together in aggregate once the solids have been placed in the MBE, so the solids will tend to wash out of the MBE following renewed injection at the injector. Another disadvantage is possible “bridging” problems of the solid materials trying to pass narrow slotted liner slots to an MBE. There is also a possibility of shallow bridging in the MBE as well, should solids successfully pass the liner into the MBE. Another disadvantage is possible settling of the solids out of the slurry before they transport very far or at all into the MBE channel.
Another method for sealing MBE's uses a gel material to seal the MBE, by pumping gel full-bore or through coil tubing to the MBE. One problem with gels is that most gels normally require a crosslinker added to the pumped gelant which later causes the gelant to “gel” to create a very viscous gel plug. Crosslinkers require certain temperatures to activate the formation of gel, and in some reservoirs the temperatures needed do not exist in range of what is required to activate the crosslinker. Gels also do not typically result in a very solid plug once set in an MBE, and thus may not offer enough shear resistance to renewed injection into the injector following an MBE sealing job. Gels can also be quite expensive and certain chemistry issues can be fairly complex to manage properly on gel jobs.
Use of cement pumping either full-bore down the tubing or through coil tubing is another method to seal an MBE. This method can provide a very shear resistant plug in an MBE provided cement can actually be placed into the MBE for any appreciable distance. Most cements are very viscous and are therefore often difficult to place very far into an MBE, thus it is quite likely that a long MBE channel may be left mostly unfilled by cement that cannot penetrate the MBE very far. Cement may also have difficulty passing thin slotted liner slots, and cement jobs can be quite expensive. Cement sets very hard as solid, so any cement that fails to exit through the liner to the MBE during pumping may result in a costly cement milling job to reopen the wellbore following a cement attempt of an MBE. Some synthetic cement blends do exist that can be pumped as a low viscosity flowing liquid, and these products set solid like cement. These cements can pass slotted liner slots more easily than conventional cements, but these products are very expensive and may also require milling if they set inadvertently inside the wellbore.
While these methods may block or isolate the fluid flow, the materials employed may not provide a complete seal, or any blockage may only be temporary, resulting in a relatively rapid reoccurrence of fluid influx.
When a well has an MBE, it is typical that all water injection entering the well will be transported to the producer via the open MBE without displacing any more oil to the producer. At this point, effective “matrix water flooding” of the oil reserves in the flood area is broken. MBE's not sealed must be isolated in some way from the rest of the wellbore in order to block entry of injection water to the MBE, if the well is to be salvaged for restoring some functional matrix water flood to regain displacement of oil reserves between the injector and the offset producer.
For example, in a tri-lateral injector well, the lateral with the MBE must be blinded off using a plug or an isolation sleeve before the other two laterals can be placed back on injection service. Until the MBE offending lateral has been isolated, no water will waterflood the two laterals unaffected by an MBE even though they are open to receive injection. This is true because the path of least resistance for injection water is to flow to the highly conductive MBE rather than divert into matrix waterflood injection patterns.
Therefore, a need exists for permanently sealing breakthroughs created during the production of hydrocarbons in a subterranean formation.